1. Field of the Invention
The invention relates to the decoding of TCMF and MF pulses for a telephone communication switching system, and more particularly for providing a TCMF and an MF decoder time shared in a single decoder.
2. Description of the Prior Art
The invention was developed for the system shown in U.S. Pat. No. 3,767,863, issued Oct. 23, 1973, by Borbas et al for a Communication Switching System with Modular Organization and Bus, hereinafter referred to as the System S2 patent.
The previous system is described in U.S. Pat. No. 3,487,173, issued Dec. 30, 1969, by Duthie et al for a Small Exchange Stored Program Switching System, hereinafter referred to as the System S1 patent. The registers and the associated decoders of the S1 System are disclosed in U.S. Pat. No. 3,678,197, issued July 18, 1972 by Panter et al for a Dial Pulse Incoming Trunk and Register Arrangement.
The S1 System has two separate and distinct types of registers. One is a basic DP (Dial Pulse) register which can be supplied with a TCMF (Touch Calling Multi Frequency) adapter to convert it into a DP and TCMF register. The second type of register is a MF (Multi Frequency) register which can only be used to receive MF tones. Both of these registers use NOR gate decoders which convert the output of the receivers into the binary hexadecimal code used by the registers.
In developing the System S2 a universal register was required. That is, one that could be strapped to receive TCMF or MF as well as still be capable of receiving the standard DP signals. In the particular arrangement of the S2 System four registers are provided in each module and therefore one card would have to contain all of the required circuitry for all four registers. A straight forward approach along the lines of the S1 System would have required both MF and TCMF decoders and control for each register, thereby requiring four of each. However, in operation once a register has been assigned to a specific type of receiver (MF or TCMF) then only one of the two decoders supplied would actually ever be used. A typical decoder requires approximately 10 integrated circuits for both decoding and control. Therefore the straight forward approach would require approximately 80 integrated circuits to be mounted on a single card. This is not only impractical but practically impossible.